Method and device for spectral level balancing in wavelength division multiplex multi-channel systems

ABSTRACT

WDB multi-channel systems for large distances having a number of intensifiers cause an increase in intensification, which can lead to considerable differences in intensity, even given modest wavelength dependence This makes spectral levelling necessary. In accordance with the invention, the light from the fiber is first split spectrally into channels, and, thus locally separated, projected onto the surface of an optic limiter based on an OASLM (optical addressed, spatial light modulator), in which the intensive channels are overproportionally attenuated The tension dependence of liquid crystal cells, derived from exposure, is used to level the WDM channels Lastly, the channels are again imaged onto a point and coupled into another fiber. The invention is used in WDM multi-channel systems for large distances with a number of intensifiers

[0001] The invention relates to a method and a device for spectral level balancing in WDM multi-channel systems as described, for example, in H. HULTZSCH Oplische Telekommunikationssysteme [Optical Telecommunications Systems], Gelsenkirchen 1996, p 344 ff

[0002] WDM multi-channel systems employ a larger number of equally spaced spectral channels for optical transmission in the glass-fiber network. The transmission capacity is increased through parallel transmission, without the transmission frequencies having to be raised, i e, a 2 5 Gbit system can be used with 20 channels for the transmission of 50 Gbit without changing the electronic components In general, WDM systems operate in the third optical window of telecommunications at 1550 nm, where there are suitable optical amplifiers capable of amplifying the entire multi-channel system, channel by channel. Although the amplifier basically does not distinguish between individual channels, the amplification curve V (λ) is dependent on the spectral position of the individual channel If a number N of amplifiers is required for greater distances, the amplification V^(N) (λ) rises exponentially, which, even given a modest wavelength dependence V(λ)·I(λ), may lead to considerable dependences of the intensity V^(N)(λ)·I_(o)(λ) at the end of the transmission link If these are no longer tolerable, suitable filters must be used to ensure a uniform spectral dependence of V^(N)(λ)I_(o)(λ) I_(o)(λ) is the intensity of the channel of wavelength λ at the input of the amplifier chain

[0003] For spectral balancing, it is obvious to employ colored glass filters or interference filters of transmittance D(λ) to level the channel intensities D(λ) V^(N)(λ)I_(o)(λ) at the output. Such filters operate completely analogously to a matching filter for the color temperature of the light source to a photographic film Their reciprocal characteristic reduces the intensity of the intense spectral channels to that of the weak ones This works with photographic film, but not in the case of a WDM multi-channel system, because the curves V(λ) differ greatly from amplifier to amplifier and are also timedependent. Actually, the intensity I_(A)(λ) at the output of the WDM transmission system is therefore described by

[0004] Such a characteristic cannot be balanced by one single static filter Even in the case of low time-dependence, the great dependence of the amplification curve V_(i)(λ) on the amplifier i and the dependence of I_(A) on the number of amplifiers N makes the development of a uniform level-balancing filter very difficult This problem is made worse by other channel-dependent dispersion and damping effects, such as polarization mode dispersion

[0005] The object of the invention is to avoid such problems.

[0006] This objective is achieved by the method presented in the characterizing part of claim 1 and by the device described in the characterizing part of claim 2

[0007] Advantageous possibilities for further development are apparent from the characterizing parts of subclaims 3 and 4.

[0008] Hereinbelow, the invention is described in greater detail on the basis of the following exemplary embodiments In the associated Drawing

[0009]FIG. 1 shows the nonlinear characteristic of an OASLM,

[0010]FIG. 2 shows a device with an OASLM between imaging gratings; and

[0011]FIG. 3 shows a device with an OASLM between fiber gratings with anamorphotic lenses

[0012] Optically addressable spatial light modulators (OASLMs) are known, for example, from “Spatial Light Modulators, OSA—Technical Digest, ISBN 1-55752-494-7 Washington 1997” and are made of a photoconducting layer and an electro-optical layer which is voltage-sensitive In response to local exposure to the light, the voltage in the photoconductor collapses locally and is transmitted to the electro-optical layer, which is thereby locally changed in its transmittance or reflection characteristic and, in turn, now indicates the exposure optically. Of course, the photoconducting layer must be sensitive to the incident light exposure; the electro-optical layer is, for example, a liquid crystal which possesses optical modulator properties in wide spectral ranges Certain materials combine the properties of the photon-sensitive and voltage-sensitive layers, such as the photorefractive crystals or polymers The layers may be patterned and resolved into individual optical points (pixels) in order to reduce the cross-talk between nearby image points or in order to obtain pixel-wise additional electrical intervention in the modulator

[0013] Certain liquid crystals in OASLMs have the property of being nonlinear; that is, their modulation properties with regard to light depend in nonlinear manner on the applied voltage and therefore on the illumination intensity I on the photoconductor FIG. 1 shows a typical characteristic of a device for local attenuation of the light intensity according to DPA 196 16 323 4 The element described therein was developed for the attenuation of strong light sources in the visible region for protection of the eye or from video cameras.

[0014] First of all, L. Wang et al. “Design of an optically addressed spatial light modulator sensitive to 1.55 μ—write light” in OSA Technical Digest Vol 9 (1995) p. 89 and, more recently, several authors in “Spatial Light Modulators, OSA Technical Digest, ISBN 1-55752-494-7 Washington 1997” p 81 and p 84 have proposed photoconducting layers for the third optical window at 1550 nm for use in OASLMs, with the result that application as an optical limiter for WDM multi-channel operation enters into consideration

[0015] As can be seen from FIG. 2, according to the invention, an OASLM sensitive to the IR light of optical telecommunication is built into a glass-fiber link with WDM such that it levels the intensity of the spectral WDM channels For this purpose, the light from the fiber is spectrally split and projected onto the OASLM, so that the spectral channels are locally imaged separately from each other The nonlinear OASLM weakens the very intense channels disproportionally and levels the intensity Subsequently, the WDM channels are imaged together again onto a point and coupled into a further fiber All types of spectral apparatuses, such as prisms, gratings or interferometers, may be used for the spectral splitting of the WDM channels

[0016] The imaging optics must also be selected depending on the type of spectral apparatus. In FIG. 2, convex holographic gratings having both spectrally splitting and imaging properties are preferably selected

[0017] The spectral channels can also be reunited by a lens for recoupling into the fiber if the apparatus conditions can be met. The important aspect is the number and splitting of the individual WDM channels.

[0018] According to FIG. 3, it is also possible to use outcoupling and incoupling gratings which can be written directly into the fiber and which may possibly be combined with suitable lenses on the fiber itself 

1. A method for spectral level balancing in WDM multi-channel systems, characterized in that the light from the fiber is first spectrally split into channels and locally projected separately onto the surface of an optical limiter on the basis of an OASLM (optically addressed spatial light modulator), that the intense channels are disproportionately weakened therein by using the nonlinear voltage dependence, derived from the exposure to the light, of liquid-crystal cells to level the WDM channels; and that the channels are finally imaged again onto a point and are coupled into a further fiber 2 A device for spectral level balancing in WDM multi-channel systems, characterized in that the device is made of spectrally splitting elements built into the fiber and of an optical limiter on the basis of an OASLM (optically addressed spatial light modulator) on the projection surface, which is built directly into the fiber between spectrally splitting elements and imaging elements and produces the proportional voltage at the liquid-crystal cell through exposure of a photoconductor 3 The device as recited in claim 1, characterized in that the spectrally splitting and imaging elements are convex gratings.
 4. The device as recited in claim 1, characterized in that the spectrally splitting elements are fiber gratings provided with anamorphotic lenses on the fiber for imaging. 